|
Seas are salty but don't get any saltier
Q:
 Why is the sea water salty--Eduardo A., Mendoza, Argentina
[A. Kalvaitis, NOAA] Pacific Ocean off southern California
A: As the rains fall and water flows over the land, the water dissolves salt out of the rocks, washes the salt into
streams, then rivers, and finally carries the salt to the sea. The salt stays in the sea because no water flows out of the
sea-just as no water flows out of the Great Salt Lake in Utah. When seawater evaporates to form clouds, almost all of
the salt stays behind. The left-behind salt slowly accumulates until, over the eons, the seas became salty-now about
three percent.
That's the simple picture, which is true but incomplete.
Seawater also picks up salt from the oceanic crust. The ocean floor has places, called hydrothermal vents, where seawater seeps into the rocks of the oceanic
crust, gets hot, dissolves salts from the crust, flows back into the ocean with its salt load, and increases the ocean's saltiness.
Volcanos erupting under the sea is yet another way the sea gets salty. Seawater, once again, dissolves salts from the molten rock.
Will the seas keep on getting salty? No. The oceans have stayed at about three percent for hundreds of million years because they lose salt in several ways.
Pick up a clamshell and heft it in your hand: heavy. All creatures need sodium to live and most need calcium to build skeletons and shells. The clam, like all
sea creatures, gets its sodium and calcium from seawater salt. When the creatures die, their salt gets locked up in sediment. Some of the sediment gets pushed
deep within Earth-more about that in a moment.
The reactions between seawater and rocks are not one way. Sea salt not only dissolves from rocks, it also reacts with the sea rocks and rocks of the ocean
crust and volcanic lava. The reactions remove some of the dissolved salts from the sea.
Plate tectonics explains the last mechanism for a balanced state of ocean saltiness. The outer hard crust of Earth consists actually of a dozen or so distinct,
hard plates that drift individually on hot, deformable rock like floating islands on a sea. An unequal distribution of heat within Earth moves the plates much
like marshmallows move on simmering cocoa.
When an ocean plate collides with a continental one, the less dense continental plate floats over the ocean one. The ocean floor gets pushed under, in the
process, and at least half its mineral-rich, salty sediments end up lost deep within Earth.
So, that's why the seas are salty but don't get any saltier.
(Answered by April Holladay, science correspondent, September 12, 2001)
Further Surfing:
U of Texas at Dallas: why is the sea salty
Alaska Science Forum, The shaky solution for a too-salty sea, Carla Helfferich
|
